mkgmap is a java program that will convert OSM data to Garmin map format. It's not perfect, but I've found it works well with my GPSMAP 60CSx.

Be sure to check out Openstreetmap.org. It is basically a collaborative mapping project where you can upload your own features in a patent unencumbered manner.

I've been mapping a lot of my local bike and wooded paths in my area using GPS.

Your area may already have this information.